1. Field of the Invention
The present invention relates to an in-cylinder fuel injection engine in which homogeneous mixture combustion or stratified charge combustion is selectively performed according to operating conditions of the engine and to the control method thereof.
2. Discussion of Prior Arts
The in-cylinder fuel injection engine is an engine in which fuel is injected directly into the combustion chamber and so-called lean burn is realized by rapidly mixing fuel with air and forming combustible air-fuel mixture around a spark plug, intending the improvement of fuel economy and the stability of combustion in light loads and low speeds condition of the engine.
A Japanese Patent Application Laid-open No. Toku-Kai-Hei 11-223127 (prior art 1) discloses an in-cylinder fuel injection engine wherein a spark plug is disposed in the midst of a combustion chamber of a penthouse-roof type and a cavity is provided on a top face of a piston in a state offset on the intake port side. In this prior art, the direction of fuel injected from an fuel injector is inclined diagonally and downwardly with respect to a center axis of a cylinder and the profile of spray distribution of injected fuel is fan-shaped so as to rapidly diffuse fuel into the combustion chamber.
On the other hand, Japanese Patent Applications Laid-open No. Toku-Kai 2000-265842 (prior art 2) and No. Toku-Kai 2000-248944 (prior art 3) disclose a piston structure in which an inner wall surface of a cavity formed on the top face of a piston is shaped into an overhang configuration. In these prior arts, when a spray block of fuel collides against the inner wall surface of the cavity, a swirl is generated by the penetration of the fuel injector. As a result, particularly under light loads, the spray block of fuel is flung up towards an electrode of a spark plug to form a locally rich air-fuel mixture around the electrode of the spark plug, whereby misfiring can be prevented and the combustion stability is enhanced.
The combustion strategy of the aforesaid in-cylinder fuel injection engine has two modes, homogeneous mixture combustion and stratified charge combustion either of which is selected according to operating conditions of the engine. When the engine operates at heavy loads and high speeds, namely, in a stoichiometric air-fuel ratio condition, a uniform mixture is formed in the combustion chamber and so-called homogeneous mixture combustion is performed. To enhance the uniformity of mixture, fuel is sprayed during the intake stroke.
On the other hand, when the engine operates at light loads and low speeds like idle speeds, namely, in a lean air fuel ratio condition (lean burn zone of air-fuel ratio), air-fuel mixture is stratified in the combustion chamber and so-called stratified charge combustion takes place. In this stratified charge combustion strategy, fuel is sprayed at a latter stage of the compression stroke. The fuel injected from the fuel injector is introduced to the cavity formed on the top surface of the piston and streams towards the electrode of the spark plug. Thus, an ignitable mixture gas is formed in the vicinity of the electrode and is ignited by the spark plug at an appropriate timing.
However, when the homogeneous charge combustion takes place in a high load and high speed operating region of the engine, the prior art 1 has a difficulty of enhancing a disperseability of fuel and an air utilization ratio. As illustrated in FIG. 12, a fuel injector 12 is disposed on an intake side of a combustion chamber 8 of an engine E and fuel is injected from the fuel injector 12 in a spray block F. Further, a cavity 5 is formed on a top face 4a of a piston 4 in a position offset from a central axis of a cylinder 1 to the intake side. A central axis Q of the spray block F shows an injection direction of fuel of the fuel injector 12, being directed to the cavity 5. Consequently, when the spray block F is injected from the fuel injector 12, almost all quantity of fuel is injected toward inside of the cavity 5.
When the piston 4 descends from the top dead center in a state while an intake port 9a is opened by an intake valve 10aand an exhaust port 9b is closed by an exhaust valve 10b, the fuel spray block F is affected by an air stream of the intake air (indicated by a bold arrow of FIG. 12). Hence, when the piston 4 goes up from the bottom dead center, the fuel spray block F is involved in the air stream of the intake air, as illustrated in FIG. 13. As a result, as shown in FIG. 14, the spray block F stays inside of the cavity 5 and more of the rich fuel tends to exist on the intake side. As a result, since the disperseability of fuel and the air utilization ratio can not be enhanced adequately, miscellaneous adverse effects such as loss of thermal efficiency, power-downing and exacerbated fuel economy and the like are caused.
On the other hand, when the stratified charge combustion takes place in a low load and low speed operating region of the engine, fuel is injected inside the cavity 5 when the cavity 5 comes close to the fuel injector 12 at the latter stage of the compression stroke. At this moment, as shown in FIG. 15, most of fuel is reflected as a main spray block F towards an inner wall surface 5a on the exhaust side of the cavity 5, however the rest part of fuel is reflected as a residual spray block F0 towards an inner wall surface 5b on the intake side of the cavity 5. As a result, the residual spray block F0 is reflected in an opposite direction of the main spray block F and both spray blocks are separated from each other. When the spark plug is ignited in this state, the flame propagation does not reach the residual spray block F0 and as a result the residual fuel spray F0 stays unburned in the combustion chamber, this incurring exacerbated exhaust emissions and fuel economy.
To solve the problem, it is considered that the ignition timing is slightly retarded. That is, during the retarded ignition timing, the main spray block F is diffused towards the intake side and joins with the residual spray block F0 in the vicinity of the intake port. The ignition takes place when both spray blocks meet together. However, when the main spray block F joins with the residual spray block F0, there is a possibility that a ring-shaped stream of the mixture gas is generated by the joining of both spray blocks. Then, a central portion of the ring-shaped stream tends to remain unburned. Further, the fuel diffused on the exhaust side also tends to remain unburned. Further, since the main spray block around the electrode 11a of the spark plug has a stream towards the intake side, the stability of ignition and the stability of combustion are exacerbated.
On the other hand, when the stratified charge combustion takes place in a high speed condition of the engine, it is difficult to find an appropriate ignition timing and as a result the thermal efficiency goes down.
Further, with respect to the prior arts 2 and 3, as shown in FIG. 17, an inner wall surface 5a opposite to the direction the fuel injection of the cavity 5 has an overhang configuration. Hence, when the fuel spray block F collides with the inner wall surface 5a of the cavity 5, the greater part of the spray block F stays within the cavity 5. As a result, the homogeneous charge combustion tends to produce a lean fuel distribution on the exhaust side.
It is an object of the present invention to provide an in-cylinder fuel injection engine enhanced in thermal efficiency, combustion stability and emissions performance when the stratified charge combustion is perfomed, and improved in thermal efficiency, power output performance and fuel economy when the homogeneous charge combustion is performed.
In order to attain the object, the in-cylinder fuel injection engine comprises a cavity provided on a top surface of a piston on an intake side, a fuel injector for injecting fuel in two directions, a first injection direction and a second injection direction and injection timing control means. That is, the fuel injector forms two fuel spray blocks, one spray block directed to the first injection direction and the other spray block directed to the second injection direction. The injection timing control means controls an injection timing so as to direct the first injection direction to the cavity and to direct the second injection direction to the top surface of the piston outside of the cavity, when the piston is during the intake stroke and positioned at a specified crank angle, preferably between 130xc2x0 and 160xc2x0, after top dead center on a bottom dead center side and for controlling the injection timing so as to direct both of the first injection direction and the second injection direction to the cavity, when the piston is at a latter stage of the compression stroke.
According to a second aspect of the invention, a fuel injector forms two groups of fuel spray blocks, a first group of spray blocks including a plurality of spray blocks and a second group of spray blocks including a plurality of spray blocks. When the homogeneous charge combustion is performed, the first group of spray blocks is injected inside of the cavity and the second group of spray blocks is injected outside of the cavity. When the stratified charge combustion is performed, both groups of spray blocks are injected inside of the cavity.